1. Field of the Invention
The present invention relates to a composite semiconductor device in which arrays such as light-emitting diode (LED) arrays are combined with driver circuits, a light-emitting diode (LED) print head that employs the composite semiconductor device, and an image forming apparatus that employs the LED print head.
2. Description of the Related Art
Among conventional electrophotographic printers is one that incorporates a light-emitting diode (LED) exposing unit. An LED exposing unit incorporates a plurality of LED array chips each of which includes a plurality of light emitting diodes (LEDs).
Many printers of late are capable of printing not only characters but also images. Printing images with high resolution places great demands on printers, leading to increasing number of LEDs used in an exposing unit. This increases the number of LED array chips and driver chips incorporated in the exposing unit.
Japanese patent Laid-Open No. 2004-179641 discloses one way of meeting these demands. Driver circuits are formed on a semiconductor substrate, and LED array chips are bonded on unused areas on the semiconductor substrate, thereby manufacturing a composite semiconductor device. The composite semiconductor devices are mounted on a printed circuit board instead of mounting individual LED arrays and driver circuits on the printed circuit board, thereby reducing the number of chips mounted on the circuit board. The LEDs on the LED array chips are electrically connected to corresponding driver circuits through electrodes formed on the bottom surface of the LED array chips.
In order to drive a plurality of LEDs formed on a single LED array chip on a time shared basis, each LED includes two electrodes, i.e., a p-side electrode and an n-side electrode wired independently of other LEDs. The two electrodes of each LED are formed on the front surface of the LED array chip, and are electrically connected to the driver circuit. The electrodes closer to light emitting regions are aligned in parallel to the row of the LEDs, and the electrodes farther from the light emitting regions are aligned beside the row of the LEDs substantially in parallel to the row of the LEDs.
Because the electrodes are formed in two parallel rows, the aforementioned LED array chip cannot be small in its widthwise direction, i.e., a direction perpendicular to a direction in which the LEDs are aligned. Therefore, the overall area of the LED array chip is difficult to make small. A thin film LED array chip is first fabricated on, for example, a GaAs substrate, and is then released from the GaAs substrate by etching. Consequently, an LED array chip having a large surface area requires a longer etching time.